1. Field of the Invention
The present invention relates to the separation of oxides of heavy isotopes of hydrogen, and in particular to a process and apparatus for separating deuterium oxide (HDO, D2O), tritium oxide (HTO, T2O) and oxides of deuterium and tritium (DTO) from light water (H2O) contaminated with heavy isotopes of water. In addition, this process addresses separation of heavy water isotopes, e.g. DTO from D2O, and HTO from D2O. Separation is effected by passing the contaminated water through a molecular separation material containing hydration sites, i.e., sites with associated waters of hydration. The heavy isotopic water is held at higher concentrations within the waters of hydration than in the contaminated water thus providing a separation effect. Heavy isotopic water can also replace adsorbed light water. Separation of the isotope molecules may also be effected with a separation membrane that selectively allows passage of light water molecules in preference to the other heavy isotope molecules. These two procedures may also be combined.
2. Description of the Prior Art
Nuclear power plants must routinely deal with the replacement and disposal of contaminated water taken from the core reactor that is laden with heavy isotopes of hydrogen, namely deuterium oxides, tritium oxides and deuterium-tritium oxides. Tritium in particular is highly radioactive having a half-life of about twelve and one half years emitting beta rays to form helium.
Periodically, the contaminated water from nuclear reactors must be replaced. It has become industry practice of dispose of the old contaminated water by simply dispersing it over adjacent ground areas or evaporating the contaminated water into the atmosphere. This is stressful to the environment as the deuterium oxides and tritium oxides are now known to have contaminated ground water sources. One alternative is to sequester contaminated water in concrete at a considerable expense.
In accordance with the present invention, a process and related apparatus are described for separating deuterium oxide (HDO, D2O ) and tritium oxide (HTO, T2O), i.e. heavy water and tritiated water, and deuterium-tritium oxides, from waste water. As used herein, water molecules of the formula H2O will be referred to as light water molecules, or simply water molecules, while water molecules in which one or both of the hydrogen atoms have been replaced by one of these hydrogen isotopes will be referred to as isotope water molecules or isotope molecules.
In the described process, a portion of the isotope water molecules are removed from contaminated water, i.e., water containing a small amount of isotope water molecules, through selective adsorption by contacting the contaminated water with a molecular separation material containing hydration sites carrying one or more associated waters of hydration. In the process, isotope water molecules present in the contaminated water selectively replace a portion of the waters of hydration associated with the hydration sites. The molecular separation material can then be separated from the water, reducing the percentage of isotope molecules in the water. After separation, the molecular separation material can be regenerated by removing the isotope molecules for long-term storage, and reused repeatedly to separate isotope molecules.
In order to improve the efficiency of the selective adsorption process, the percentage of isotope molecules in the contaminated water can be increased, thereby increasing the exposure of isotope molecules to hydration sites, by removing a portion of the light water molecules, before or during the selective adsorption, by bringing the contaminated water into contact with a porous film or membrane that exhibits a greater permeability for light water molecules than for the larger isotope molecules. For some purposes, adequate separation may be effected through membrane separation alone.
The Molecular Separation Material
Generally, the molecular separation material of the present invention is comprised of a support medium having a plurality of hydration sites, i.e., sites with associated waters of hydration. The effectiveness of the molecular separation material is determined by the number of hydration sites exposed to the contaminated water, and to the number of waters of hydration at each site. The support medium used to carry the hydration sites is not critical to the invention so long as exposure of the contaminated water to numerous sites containing multiple waters of hydration is provided. In general, this objective is preferably achievable with a high surface area support medium having a plurality of hydration attachment sites.
The support medium or medium may be, for example, a polymer, such as polystyrene/divinyl benzene (PSDVB), or polyacrylic/divinyl benzene (PADVB). These polymers are commonly used as supports in ion exchange resins in the preparation of ion exchange resins. The polymer may be functionalized for example, by being sulfonated or phosphonated to provide the sites for attachment of metal or other cations with the required associated waters of hydration. Both strong and weak acid resins have been shown to be effective.
It is important to note that the present invention involves the preferential adsorption or substitution of the waters of hydration associated with the hydration sites, and not the replacement of the cation or anion as is normally practiced in using this type of resin. Thus, while the resins employed are referred to in some instances as ion exchange resins, since this is the purpose for which they are commonly employed, their function in the present invention is to facilitate molecular exchange of isotope water molecules with the associated light water molecules attached to the hydration sites.
Also, while the present invention will be exemplified by the use of the above resins, it will also become apparent that other materials having a large surface area and hydration sites can be used. That is, the present invention involves the interaction between the hydration sites and the isotope molecules, in which one or more light water molecules initially associated with a hydration site are replaced by isotope molecules in the contaminated water. Thus, the support medium serves essentially as a carrier for the hydration sites. Thus, various high surface area materials can be used, so long as they are water insoluble and provide a large number of accessible hydration sites. For example, the support medium can be other kinds of synthetic polymers, or natural materials, such as zeolites, aluminas, silicas, etc.
Each hydration site will have at least one, and preferably from about 7 to about 25 waters of hydration and even higher up to almost 50 waters of hydration. Various molecules that form associations with water molecules, i.e., waters of hydration can be used in the present invention. The cationic portion of the hydration site may be non-metallic, e.g., an ammonium cation (NH4+), or a metallic cation. Of the metal cations, aluminum is especially suitable due to the large number of waters of hydration associated with aluminum salts. However, other cations, such as sodium, magnesium, copper, zinc, cobalt, iron, nickel, manganese, potassium or chromium can also be employed. Depending upon the structure of the support and the manner of its production, the anionic portion of the hydration site molecule can include nitrates, sulfates, chlorides, acrylates, hydroxides, or phosphates. Moreover, a broad array of physical constants for inorganic compounds having varying waters of hydration are to be found in reference handbooks such as Handbook of Chemistry, N. A. Lange, Ph.D. Revised 10th Edition, or CRC Handbook of Chemistry and Physics D. R. Lide, Ph.D., 77th Edition.
The molecular separation material may be in various physical forms, so long as a large surface area with hydration sites is exposed to the contaminated water. For ease of manufacture and subsequent regeneration, and the availability of a large surface area, the molecular separation material is preferably in the particulate form., e.g. beads of from about 15 mesh to about 400 mesh. Other physical forms, such as gels, can also be used.
The Separation Membrane
Separation of the isotopes may also be effected with the use of a separation membrane, or a separation membrane may be used simultaneously, or in sequence with selective adsorption. Suitable separation membranes have a porosity that is selective for light water molecules. That is, the membrane will allow a greater percentage of light water molecules than isotope water molecules to pass through the membrane when the contaminated water is placed against one side of the membrane. The separation membrane may be formed of various materials, such as cellulose acetate. Other suitable separation membrane materials will become apparent to one skilled in the use of such materials for molecular separation.
When used, the waste or contaminated water is passed against one side of the membrane surface, causing light water molecules, and a relative small percentage of isotope water molecules to pass through the membrane wall. As a result, the percentage of isotope water in the remaining contaminated water is increased. Therefore, the membrane can be used alone to reduce the volume of the contaminated water for subsequent storage, or to concentrate the isotope for treatment with the above-described molecular separation material.
The separation membrane may be positioned for contact with the contaminated water in various ways known to one skilled in the art of using separation membranes, so long as the contaminated water can be conveyed on one side of the membrane, with the light water molecules being permitted to pass through the membrane to the opposite side. Other conditions being the same, the permeation rate of the membrane is directly proportional to the surface area of membrane exposed to the contaminated water.
A preferred configuration for purposes of the present invention is to use a separation membrane in the form of one or more hollow fibers, with the contaminated water being passed through the interior of these fibers. As a result, the light water molecules preferentially pass through the walls of the fibers to the exterior of the fibers for collection.
The separation membrane may be used in combination with the above molecular separation material for sequential or simultaneous water treatment. For example, the contaminated water may be first exposed to the separation membrane to remove a portion of the light water, thereby concentrating the contaminated water stream. The concentrated stream can then be exposed to the molecular separation material, thereby increasing the effectiveness of the molecular separation, since the isotopes comprise a relatively higher percentage of the waste stream.
Alternatively, the contaminated water may be simultaneously subjected to membrane and molecular separation. For example, the membrane can be in tubular form, e.g., lengths of hollow core fiber, and the molecular separation material can be packed into the interior of fiber or tube. The contaminated water can then be conveyed through lengths of the filled tube or hollow core fiber, discharging substantially purified water therefrom with the isotope water molecules, i.e., the oxides of heavy isotopes of hydrogen, being held or trapped within the tube or hollow core fiber for appropriate disposal or regeneration.
Thus, in one embodiment of the invention, the heavy water or tritiated water content of a contaminated water stream is reduced by exposing the stream to a single elongated length or a bundle of hollow core fibers, each of which is at least partially filled or packed with beads of an exchange resin, or other molecular separation material.
Apparatus and Process
The configuration of the apparatus used to practice the process of the invention will vary depending on whether the molecular separation material, the separation membrane, or both, are used. The exact nature of the apparatus will also depend upon the volume of water being treated, the manner of disposal of the water discharge streams, and whether or not the molecular separation material, if used, is to be regenerated.
In general, however, the apparatus will include at least one separation chamber, a supply conduit for conveying contaminated water into the separation chamber from a supply source, and a first discharge conduit for removing treated contaminated water from the separation chamber. For example, when the molecular separation material is used alone, the apparatus may include a separation chamber to hold the molecular separation material, a conduit to feed contaminated water into the separation chamber from a supply source, and a discharge conduit for removing treated water from which a portion of the isotope molecules has been removed. Provision may also be made for periodic replacement of the molecular separation material.
The apparatus may also include a means for regeneration of the molecular separation material to remove adsorbed isotope molecules and regular water molecules. For example, the loaded molecular separation material can be placed in a heated chamber to drive off the isotope molecules and the light water molecules by evaporation. This desorbed or dehydrated molecular separation material can then be used directly, or rehydrated with light water molecules prior to use.
When the separation membrane is used alone, the apparatus will also include a separation chamber in which the contaminated water is passed on one side of the membrane. The apparatus will also include a supply conduit, a first discharge conduit for conveying the treated water passing through the membrane, and a second conduit for conveying the remaining concentrated water. When the separation membrane is in tubular form such as a hollow fiber, the first discharge conduit is in communication with the exterior of the tubes or fibers, while the second discharge conduit is in communication with the interior of the tubes or fibers.
The two types of apparatus can be joined together for the combined treatment of the contaminated water with the molecular separation material and the separation membrane. For example, a supply conduit can convey water from a supply source to a first treatment chamber containing the separation membrane. Concentrated water from this first stage treatment can then be conveyed to a second separation chamber holding the molecular separation material.
Thus, in one embodiment, the percentage of isotope water molecules in water is reduced by the steps of (a) conveying water containing a percentage of isotope molecules into contact with a molecular separation material having a plurality of hydration sites, (b) substituting or hydrating a portion of the waters of hydration with isotope water molecules, and (c) separating the molecular separation material with associated isotope waters of hydration from the contaminated water.
In another embodiment of the invention, isotope water molecules in water is reduced by the steps of (a) conveying water containing a percentage of isotope molecules into engagement. with one side of a permeable membrane, that allowing selective passage of light water molecules in preference to isotope water molecules, whereby light water molecules and a relatively minor percentage of isotope molecules pass through the membrane, and (b) collecting the concentrated water that did not pass through the membrane.
In the combined process, isotope water molecules in water are reduced by the steps of (a) conveying water containing a percentage of isotope molecules into engagement with one side of a permeable membrane, that allowing selective passage of light water molecules in preference to isotope water molecules, whereby light water molecules and a relatively minor percentage of isotope molecules pass through the membrane, (b) conveying concentrated water that did not pass through the membrane into contact with a molecular separation material having a plurality of hydration sites, (c) substituting a portion of the waters of hydration with isotope water molecules, and (d) separating the molecular separation material with associated isotope waters of hydration from the contaminated water.
Each of the above processes may include additional steps. For example, the first or combined process may further include the steps of (a) regenerating the molecular separation material to separate at least some waters of hydration, (b) collecting isotope water molecules separated from the molecular separation material, and (c) returning the regenerated molecular separation material, with or without rehydration, to the separation chamber.
The present invention is presumed to be based upon a molecular exchange principle of either adsorption or selective adsorption to accomplish the experimental results reported herebelow. Although the co-inventors herein differ on the precise theory of the operation, it is understood that the test results below speak for themselves with respect to the efficacy of the various embodiments of the invention.
It is therefore an object of this invention to provide an environmentally safe alternative to the ground or air dispersion of water contaminated with heavy isotopes of hydrogen.
It is yet another object of this invention to provide means for separating heavy isotopes of hydrogen from light water (H20) and tritiated water from heavy water.
It is still another object of this invention to provide a commercially viable apparatus containing a bundle of filled hollow core fiber lengths in a housing for separating heavy isotopes of hydrogen, including tritium, from contaminated water and a method for regenerating said apparatus.
It is another object of the invention to provide a process for separating isotope molecules from water by contacting the water with a molecular separation material that includes hydration sites with associated waters of hydration and methods of regenerating same for reuse.
Another object of the invention is to provide a process for separating isotope molecules from water by concentrating the isotope molecules using a separation membrane, and contacting the concentrated water with a molecular separation material that includes cation sites with associated waters of hydration and regeneration thereof.
In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with reference to the accompanying drawings.